stumbling / bucking

[bobt]

My 1915 touring is stumbling / bucking between 25-30 mph right where I like to cruise in high gear. I've been through the NH carb, new valves and Neway grind. (I did not replace the valve springs? Maybe weak?) No fuel filter. Potato and screen are clean. I've had the timing and carb jets from one extreme to the other. Coils are good per ECCT. Compression about 35 Shorted out each plug and It's hitting on all four. Cleaned New Day timer and checked firing order. plenty of fresh gas. bobt

[rickd]

Bob, are you running on Mag? Check AC volt mag output, check the mag post for debris stuck to it?

[TXGOAT2]

Weak valve springs could allow valves to float or bounce at certain speeds. Problems with the wiring between the coils and timer will cause problems. Poor contact between the coils and the coil box contacts is a common problem. Vibration at certain speeds can make it worse. Misalignment, a weak brush spring, or a badly worn brush can cause timer problems.

[bobt]

Magnito or 12 volt battery- it does the same. new brush and spring on cleanly sanded new day timer. The timer harness was replaced last year. It idles fine when I have the carb and timing adjuster to where I've always ran it. I've also changed the spark plugs. Thanks, bobt

[KWTownsend]

Carbon tracking in the coil box?

[TXGOAT2]

There is no fixed, correct setting for the timing or the carburetor mixture. The proper setting is where the engine runs best at whatever speed and load it is running at. Float level setting is important, and in some cases, the float needle and seat assembly won't deliver enough fuel for running at road speeds. Float drop may be an issue at road speeds. The gas cap air vent must be clear and not obstructed by anything near it, like the seat cushion or items under the seat.

[TXGOAT2]

The engine's demand for fuel is much higher at road speeds than at idle, and the voltage demand on the ignition system is much higher at higher engine loads than at idle. Valve clearance that is too tight may allow a good idle, but cause problems when the engine is running at higher speeds and loads.

[Steve Jelf]

The symptoms in this case always remind me that Scott Conger makes correct NH float valves for a reason. Check your valve's opening. Is it .100", or .125"?

[Moxie26]

Better yet, .... Is the float set correctly for the carburetor he's using?

[Norman Kling]

I haven't heard of anyone using a gros jet in the carburetor float, but many years ago my carburetor had one and it would not pull hills very well or run at higher speeds well. This jet looks like a ball bearing rather than a needle valve. My car would run out of gas going uphill and if I waited a bit then it would start again. With gravity feed fuel system there is not much pressure for the gas to push through the valve and starves for gas. I replaced with a standard float valve and have not had problems since. Another problem is from old gas in the car which turns like varnish which will clog the fuel line and carburetor jets. If this happens you need to be clean it out and fresh gas put in the tank.
Norm

[bobt]

Thanks for everyone's input. I'll let you know if I ever find out the problem. bobt

[MKossor]

New spark plugs doesn't eliminate the spark plug variable based on my experience. I installed brand new, expensive, properly gapped Champion X spark plugs on a completely rebuilt engine with known good coils, all new wiring, including new plug wires and Delrin plastic coil box so spark from each coil got to its proper spark plug and no where else. Running an E-Timer that eliminates ALL ignition variables associated with the coil points and timer. The engine ran like Crap! WTH!

After checking everything twice and trying the old carburator that ran fine you years I was runnung out of ideas. Then my Son made a ridiculous suggestion; Why don't I try the old Champion X spark plugs? I immediately balked at started to rant they are old original plugs full of carbon, worn electrodes and a waste of time but just to show you.... I will put the old plugs back in. Yep, the engine ran great! That was it! Never figured out the technical reason why but still have those plugs and still looking for a pressurized spark plug tester to understand why that happened. Some have reported this before and resolved the issue by opening up the gap to 0.035" rather than the recommended 0.025" but haven't tried that yet. In the end, the toughest part was wiping all that egg off my face

[Jerry VanOoteghem]

Why don't I try the old Champion X spark plugs? I immediately balked at started to rant they are old original plugs full of carbon, worn electrodes and a waste of time but just to show you.... I will put the old plugs back in. Yep, the engine ran great!

Same experience here.

[Jerry VanOoteghem]

Robert,

Did the car ever run well for you? Is this a new problem?

[love2T's]

FWIW believe it or not I had similar problem on my former '15. New coil box kit, Fun Projects of course. Running on Champ X's.
Switched to Champ 25's, ran great. Just to prove the point, went back to X's, problem returned. Put the 25's back in, problem solved. You figure it out, cause I couldn't!! JTS, to the OP, I would rule out carbon tracking for sure, if for no other reason the odds are the coil box is already old as dirt anyway like most are, you can confirm of course, and could use a "freshening up" anyway. It sure can't hurt any. Just make sure you get the good quality brass contacts not the cheap flimsy ones with no "split" in the tabs that make the contact. They're junk.

[TXGOAT2]

Problems with motor vehicles are often what you're sure they're not.

[TXGOAT2]

Try warming the car up, then put it in "neutral" (brake handle all the way back) and raise the engine speed to about what it is when the car runs rough on the road, with the timing and carburetor at the same setting as when on the road. Does it still run rough?

[Moxie26]

With any spark plugs for the Model T, check for continuity between the center electrode and the top screw terminal. The ones that don't fire, are the ones that do not show continuity....

[bobt]

I've ALWAYS ran Autolite 3095. That's what it came with when I bought it years ago and it ran great. I drive my 15 and 26 A LOT. Summer and winter. I Don't go far from home and rarely drive over 25 mph. I plan on doing a leak down test within a week. bobt

[Moxie26]

BobT ..... New spark plugs of any brand can be defective, wouldn't be a problem if an analog meter was used to test each plug for continuity before installation.. this would eliminate a potential problem.

[Steve Jelf]

If you want to really test spark plugs, see how well they work under compression.

[TXGOAT2]

Don't use resistor-type spark plugs on a Model T.

[bobt]

I am reading and listening to all comments. The Autolite 3095 is not a resistor plug. Steve, when I was much younger, the shop I worked in had one of those Champion spark plug tester. Ours was smaller and also had a sandblaster on it for cleaning the plugs too. An older mechanic showed me one time why he wouldn't use Champion spark plugs in large Chrysler engines. The spark would be intermittent at about 100 pounds.

[CatGuy]

Don't use resistor-type spark plugs on a Model T.

This is something I hadn't heard of before. How can I tell the difference in spark plugs?

[Steve Jelf]

https://www.ngk.com/what-is-a-resistor-spark-plug

[TXGOAT2]

The wires connected to the timer take quite a bit of abuse from vibration and rubbing on other parts, often including the fan belt or crank pulley. Worn spots in the insulation can develop quickly and cause misfire, often at higher road speeds. It's worthwhile to check the timer wires very carefully, especially in the area near the timer and anywhere the wire loom is clamped to the engine or in contact with it. Another common problem is the coils not making secure electrical contact inside the coil box. This can usually be corrected with cardboard shims. It's a good idea to keep the high tension plug wires away from one another and from engine surfaces, and to keep them away from the timer wiring. It's commonly recommended to keep the high tension wires at least an inch from one another or anything else. Problems with the ignition switch and related wiring can cause misfiring at certain speeds.